1. Field of the Invention
The present invention relates to a device for driving a semiconductor laser which is used to record image information on a photographic film or the like.
2. Prior Art
As is well known the art, an image may be recorded on an image recording medium using a laser beam which is modulated by an image signal that represents the image. Generally, there are two methods of modulating the intensity of the laser beam with the image signal. According to one method, the intensity of the laser beam is modulated by varying the magnitude of an electric current supplied as the image signal to the semiconductor laser. This method is referred to as a pulse amplitude modulation process. The other method varies the period of time in which the electric current is supplied to the semiconductor laser, thus modulating the laser beam intensity. The latter method is referred to as a pulse width modulation process.
In order to obtain a multiplicity of image gradations on the photographic film through only the pulse amplitude modulation process, it is necessary to utilize spontaneous emission of the semiconductor laser. However, the focused spot of the laser beam on the photographic film tends to be distorted.
For producing multiple image gradations through only the pulse width modulation process, since it is necessary to reproduce a highlight area on a negative photosensitive medium in a range of gradations, the intensity of the emitted laser beam needs to be controlled stably in a low intensity range. Stated otherwise, narrow pulses of the current supplied to the semiconductor laser are required to be controlled stably. Such stable intensity control, however, is practically infeasible since it can be achieved only when the semiconductor laser drive device incorporates expensive circuit components.
In view of the above drawbacks, the applicant has proposed a technical approach to the generation of multiple image gradations as disclosed in Japanese Laid-Open Patent Publication No. 59(1984)-119960. The disclosed solution makes it possible to modulate the intensity of a laser beam for a range of image gradations through a combination of the pulse amplitude modulation process and the pulse width modulation process.